Boron is used in various manufacturing industries of electroplating, enamelware, glaze, clay roofing tile, electrical part, and the like, and waste water discharged in the manufacturing processes of those products contains a large amount of boron. However, a large amount of boron is known to exert harmful influences on environment and to cause symptoms of poisoning on the human body, such as gastrointestinal disorder, rubedo, and central nervous system symptom.
Therefore, the Water Pollution Prevention Law was revised in June, 2001, and, in addition to the conventional regulation of the discharge of water containing heavy metals and organochlorine compounds, the regulation of discharge of water containing boron started. The standard for discharge of water containing boron and its compounds is set below 10 mg per L of water (below 10 ppm) in freshwater environments such as a river, and below 230 mg per L of water (below 230 ppm) in seawater environments.
Conventionally, the following methods have been employed for treating boron-containing waste water: a method in which magnesium oxide in an amount 5 to 10 times or more (molar ratio) as large as that of boron is added to boron-containing waste water, the pH is adjusted to 10 or higher with alkali hydroxide to form an insoluble precipitate, and then the formed insoluble precipitate is removed; a method in which a large amount of aluminum salt is added, the pH is adjusted to 10 or higher with slaked lime to form an insoluble precipitate, and the insoluble precipitate is removed similarly as in the above-mentioned method; a method in which zirconium oxide in an amount equivalent to that of boron is added to form an insoluble precipitate, and the insoluble precipitate is removed; and a method in which a boron-selective ion exchange resin is brought into contact with boron-containing waste water to remove boron by adsorption.
However, the methods in which an insoluble precipitate is formed using magnesium oxide or aluminum salt for removing boron each require a large amount of a chemical agent and generate a large amount of precipitate. Thus, the methods were not economical. Moreover, the number of remaining years to use final disposal sites is limited, and hence generation of large amounts of wastes should be avoided. According to the method of removing boron using zirconium oxide, boron can be removed with a smaller amount of a chemical agent, and the amount of generated precipitate is smaller as compared with the above-mentioned methods, but there was the following problem. The solid-liquid-separation properties of the generated sludge are low, and moreover, because zirconium is a rare metal, such a method is not economical. The method using a boron-selective ion exchange resin also requires a large amount of an expensive boron-selective ion exchange resin. Thus, the method was not economical.
Under such a technical background, methods of removing boron that are performed simply and with low cost and that have sufficient effects have been studied. For example, there has been reported a method of removing boron in a solution using a gel-like polyvinyl alcohol (Patent Document 1). However, according to this method, boron is adsorbed in the vicinity of the surface of the gel-like polyvinyl alcohol for separating a boron-binding gel. Thus, although a large amount of a chemical agent is used, the amount of boron that can be bound is small. Thus, there was the following problem. In order to remove boron from water containing boron at high concentration and to reduce the concentration of boron 10 ppm or lower by using this method, a large amount of a chemical agent and a prolonged reaction time are required. Moreover, a large amount of calcium aluminate sulfate, calcium sulfate, calcium aluminate, or ferric hydroxide should be blended in the process of gel formation in order that a large amount of boron is bonded. Therefore, there was a problem in that it was impossible to reduce the amount of a boron-treating agent and a large amount of sludge was generated. In addition, the strength and manner of use of the gel need to be adjusted, and there were problems of lowering of the boron-treating ability due to drying of the gel and the increase in cost of preservation and transport for preventing the lowering. Under such circumstances, it has been desired to develop a method of removing boron capable of sufficiently removing boron with a small amount of a chemical agent and capable of suppressing the amount of generated sludge, and an agent for removing boron which is used for the method and is easy to handle.    Patent Document 1: JP 2002-186976 A